Until the launch of micius satellite by china [1]}, entanglement distribution was only possible with distances not more than 100km [2]}. But with this launch, it became possible to achieve entanglement distribution between two far away ground stations located on earth with distances greater than 1000 Km through terrestrial free space channels [3]}. Much of the system fidelity depends on the quantum channel loss, which comprises of beam diffraction, pointing error, atmospheric turbulence, and absorption. Thus, satellite based entanglement distribution puts more stringent requirements for link efficiencies than any classical satellite to ground communications. So, various techniques are heralded to achieve efficient long distance communications. Free space channel link has been demonstrated in various experiments [4]}. DARPA QKD network using free space link has been demonstrated in [5]}. Quantum communication has been demonstrated by the Zeilinger group between two peaks in the canary islands, separated by 144 km [6]}, [7]}. The free-space optical quantum channel has all the earmarks of being very promising for long-distance quantum Communication. The information-carrying photons are quick and vigorous, and are prone to environmental loss. The advantage of this approach is that most of the photon loss mechanism occurs lower than 10 Km of atmosphere, leaving behind most of the photon path as vacuum with negligible loss of photons and decoherence. Only dispersion effect is present once light leaves the atmosphere [8]}. Thus free-space optical channel keeps the quantum information in photons intact and increases the system fidelity and channel capacity. The free-space optical channel is far better than optical fiber-based channels and was first utilized for QKD in [9]}, [10]}, and is most promising channel for satellite-based long distance communication on a global scale.
